The present invention relates to a one-dimensional image sensor which receives a reflected light from an original document that has been irradiated with light and converts it into an electrical signal using a photodetector, and is applied to an image reading apparatus such as a FAX or a scanner.
FIG. 9 shows a circuit diagram of a contact type one-dimensional image sensor IC used for a conventional reading apparatus of a FAX. As shown in FIG. 9, it is a system in which an output of a phototransistor PTR is read out to a common signal line LSIG by sequentially turning on switches S1 to Sn.
The contact type phototransistor one-dimensional image sensor as described above is disclosed in Japanese published unexamined utility model application No.S61-124171.
However, in such a one-dimensional image sensor, since carriers remaining in a base of the phototransistor PTR are removed through an emitter, there has been the problem that a base potential can not be sufficiently reset to an initial state, and a residual image is serious. Besides, in order to obtain an effective bright-time output, a light source is placed in an OFF state to make a dark state prior to reading of an original document, a dark-time output is read out, and its value is recorded, and then, the light source is placed in an ON state to irradiate the original document with light, a bright-time output is read out, and a difference from the dark-time output previously recorded is obtained, so that the effective bright-time output is obtained. However, in this system, there has been a problem in that a memory for recording the dark-time output must be provided, so that an increase in costs is caused. Besides, in many cases, the previous dark-time output is not set at any time, but is set only one time when installed in a final product such as a facsimile and is shipped from a factory, and if the dark-time output has temperature characteristics, there has been also a problem in that a dark-time output recorded at the time of shipment from the factory is different from a dark-time output when an original document is actually read, so that an effective bright-time output is not correctly obtained and reproduced image quality becomes deteriorated.
In order to solve such conventional problems, an object of the present invention is to provide an image sensor with a low cost and low residual image, and to obtain an effective bright-time output with excellent accuracy.
In order to solve the above problems, according to the present invention, an image sensor is structured as follows:
(1) In an image sensor constituted by linearly arranging a plurality of linear image sensor ICs, an image sensor IC is structured such that with respect to the output of a plurality of photodiodes, an image signal changed according to an exposure amount, and an initial signal at the time when the photodiode is returned to an original state are held for each of the light receiving elements through a first amplifier connected to each of the photodiodes, and then, the image signal and the initial signal of each of the light receiving elements are sequentially input to a second amplifier through read-out switches, and its output signal is read out as a signal of a unit light receiving device.
(2) When the image signal is held, the read-out switch is placed in an OFF state, and when the initial signal is held, the read-out switch is placed in an ON state, and the initial signal is applied to an input gate of the second amplifier.
(3) When the initial signal is held, the read-out switch is placed in an ON state, and also, the second amplifier is placed in an operating state, and the initial signal is applied to the input gate of the second amplifier.
(4) The second amplifier becomes in an operating state when it reads out the signal from the light receiving element corresponding thereto, and is made to perform an operation to suppress a consumed current in almost all other periods.
(5) When the signal is read out to the outside, for each of the light receiving elements, the initial signal is output, and then, the image signal is output, and this operation is sequentially performed for all the light receiving elements.
(6) The first amplifier becomes an operating state when it holds the signal from the light receiving element corresponding thereto, and is made to perform an operation to suppress a consumed current in almost all other periods.